This invention relates generally to the continuous xanthation of alkali cellulose and more particularly to a method for controlling the oxygen level in the reaction vessel during the continuous xanthation of alkali cellulose.
Preparation of viscose for use in the manufacture of regenerated cellulose films involves a series of steps. One step in its manufacture is xanthation, wherein ripened alkali cellulose is converted into caustic soluble xanthate by treating it with carbon disulfide. In the continuous process, a closed reaction vessel is used to which the alkali cellulose is passed for continuous conversion to cellulose xanthate. The operating procedure requires the continuous feeding of alkali cellulose onto several transporting means such as two continuous belts located inside the reaction vessel. As the alkali cellulose is discharged into the vessel from the feeder onto the upper belt, it is wetted with a spray of carbon disulfide and carried the length of the xanthator vessel where it is discharged to the lower belt and returned the opposite direction for the length of the vessel. During the period of time the alkali cellulose is on the moving conveyor belts, the xanthation reaction proceeds. The cellulose xanthate is then discharged from the bottom belt into a paddle mixer to which a continuous stream of caustic and water is added forming concentrated viscose slurry which is thereafter continuously removed from the reaction vessel.
A temperature of approximately 25.degree.-35.degree. C is maintained during the reaction and the atmosphere within the xanthator reaction vessel is maintained to contain from about 25-35% carbon disulfide and about 65-75% nitrogen. Oxygen is also present in this atmosphere being carried into the unit with the air entrapped in the alkali cellulose. An oxygen level above 5% produces a dangerous atmosphere which is potentially explosive, and, in order to maintain a safe operation, the vessel is continually purged with nitrogen to reduce the oxygen level to safe limits. In the prior art methods, control of oxygen is accomplished by metering nitrogen directly into the xanthator vessel at one or more places in the tank. This method of oxygen control is satisfactory when operating speeds and alkali cellulose throughput rates are low. However, where greater production is required, excessive amounts of nitrogen must be metered into the unit to maintain an oxygen level below 5% since high throughput rates contribute to an increased oxygen level in the vessel because of the greater amounts of air being carried into the vessel with the alkali cellulose. A typical apparatus and method of the above described type is disclosed in U.S. Pat. No. 2,174,056. Such method and apparatus are effective for continuous xanthation where low throughput rates are used. However, the method is unsatisfactory where higher amounts of cellulose crumbs are introduced into the vessel if faster throughput rates are to be achieved without reaching a dangerously high level of oxugen in the atmosphere within the vessel.